1. Field of the Invention
The invention relates to the chemistry of halogenated compounds.
2. Description of Related Art
As chemical products are replaced, become outdated, or are banned for certain uses, stockpiles of potentially hazardous or environmentally harmful chemicals may accumulate. Even the storage of such materials may become potentially hazardous if storage methods are compromised. Ideally, such chemical waste should be broken down or degraded into harmless compounds. However, known degradation process are often expensive and/or may produce hazardous by-products.
Halogenated compounds constitute a growing percentage of such chemical waste. For example, halocarbons present particular problems since the by-products released during their destruction are often dangerous halogens.
One type of halocarbon, chlorofluorocarbons (CFCs), represents one group of chemicals that is becoming outdated. Since Dec. 31, 1995, manufacture of CFCs has been banned. Prior to that, market demand for reclaimed/recycled CFCs had already dropped significantly (see Heating, Piping, and Air Conditioning, 65 (4), p. 39, 1993). The result is an existing stockpile of CFCs that cannot be used. The stockpile may cause serious damage to stratospheric ozone if allowed to escape into the atmosphere.
As an alternative to destruction, some CFCs can be used as intermediates in the production of hydrochlorofluorocarbons (HCFCs). However, the HCFCs are also scheduled for a manufacturing ban in the year 2030. Destruction, then, may be the best and most desirable approach for CFC suppliers to eliminate existing stockpiles of CFCs, and there is a need for efficient methods to destroy such stockpiles.
Commonly used methods for destroying halocarbon compounds employ incineration. Although theoretical destruction efficiencies exceeding 99.9% may be achieved through conventional incineration, there are some practical difficulties in maintaining the necessary time and temperature to destroy many of the more stable species. Furthermore, incineration produces by-products such as HF or HCl which are themselves toxic and must be removed. Products of incomplete combustion present an even bigger concern since the potential to produce substances such as polychlorinated dibenzo-p-dioxins and dibenzofurans exists 5, 7!.
As noted, incineration alone can be used to destroy some percentage of the halocarbons. But reactants have also been used or added to incineration or destruction processes to attempt to make the destruction reaction more efficient. For example, the use of sodium-containing compounds or alkali metal-containing compounds has recently been discussed. (See, for example, European Patent Application No. EP 467 053; Chemical & Engineering News, vol. 74, no. 4, pp. 6-7 (1996); Science, vol. 271, pp. 340-341 (1996), and Ind. Eng. Chem. Res., vol. 28, pp. 1055-1059 (1989).)
Generally, reactants added to a destruction reaction chamber are in liquid or solid form. For example, metallic sodium in liquid or molten phase has been used. (U.S. Pat. Nos. 4,465,590 and 5,545,390 and PCT application no. WO 94/03237.) The dispersion by chemical reaction process, in U.S. Pat. No. 5,108,647, also discusses combinations of liquid and solid phase reactants.
Fitzpatrick et al., U.S. Pat. No. 4,029,484, discusses using a mist of liquid droplets of aqueous acid or an aqueous acid salt in processes for removing waste halides. The waste halides considered in the document do not, however, encompass halocarbons.
Lalancette et al., U.S. Pat. No. 4,631,183, discusses a process for destroying halogenated organic compounds involving addition of solid carbonate or bicarbonate of an alkali metal or alkaline earth metal under reductive reaction conditions at temperatures of at least 1000.degree. C., i.e. from 1000.degree. C. to 1600.degree. C. (see Col. 3, lines 32-33), whereby alkali or alkaline earth metal vapors apparently are produced "in situ". This process also produces toxic gaseous components that must be further handled or treated (see Col. 2, lines 39-42).
Other uses of sodium have been discussed. PCT Application no. WO 94/0327, describes a process involving passing CFC vapors through a bath of molten sodium. Reaction products are formed in the bath and must be separated from the molten sodium. Another method involves passing CFCs through powdered sodium oxalate, producing sodium chloride, sodium fluoride, and carbon dioxide 3!. This process has the advantage of bypassing the effects inherent in the incineration or plasma methods, but the process is expensive in both material and time. (See also Science, 271, p. 340) For example, sodium oxalate is quite expensive, as discussed in Chem. & Eng. News Vol. 74, No. 4. Additionally, the sodium oxalate process produces carbon dioxide, a greenhouse gas.
There are other, currently used processes for the destruction of CFCs, including those discussed in Refs. 1-6. For example, solid supports or matrices have been used to hold the sodium-containing compounds through which the halocarbons are passed. In addition, electronic processing methods have brought plasma destruction to the industry's attention. However, plasma destruction also results in volatile products, and high destruction efficiencies have not been proven 6!.
Despite the efforts of the prior art, there remains a need for an efficient and cost-effective process for destroying halocarbons, particularly one which lends itself to use of a streamlined, continuous feed reactor.